When you're using the MailChimp provided signup forms, make sure you've enabled the option for your recipients to choose which version of your campaign they want to see.
You'll see this option when you create a new list or under List > Settings > List Name and Defaults for existing lists. If your recipients choose Mobile, they will see the mobile version when you send your campaign, etc.

Related Articles
- I added a large image and the body color extended throughout my campaign. What gives?
- How do I delete more than one campaign?
- Can I sort my campaigns by the date they were sent?
- What can I do if images are loading slowly in my recipients’ inboxes?
- Will search engines be able to find my campaign archives?